In a metal mold for tire formation to be used for a tire vulcanization process, a number of fine ventilation holes, referred to as vent holes, have been formed. The vent holes are formed to vent air remaining between an inner surface of the metal mold and an unvulcanized tire during vulcanization of the tire, and to vent gas generated during vulcanization, to the outside of the metal mold.
A number of the vent holes are usually provided at portions in which air tends to remain in the metal mold, and provided at eight portions or more on the same circumferences of the metal mold corresponding, for example, to a bead ring, a rim cushion, a side portion, and the like. The number of circumferences differs by the shape of tire. Further, a vent hole of the metal mold in the design block which is surrounded by grooves of the tire tread is processed and provided at one portion or more.
However, when a tire is vulcanized using a metal mold provided with a number of vent holes, vulcanized rubber flows into a number of the vent holes, and thus, a number of hair-like protrusions which are a number of protrusions so-called spews are generated on a surface of a product tire.
Because of this, in a tire finishing process after the tire formation, an operation to cut off the above spews is required. There arise problems that the operation not only deteriorates workability and productivity but also worsens working conditions due to a number of scattered scraps of cut off spews, and also causes a difficulty in enhancing the external appearance of the product tire.
Thus, in recent years, there have been requests for a reduction of spew scraps and for an improved environment from the reduction of the spew scraps, and a demand for an improvement in the appearance of a tire. For those reasons, a venting apparatus made not to generate the spews the surface of a tire just after vulcanization, and a metal mold for vulcanization with a valve provided in an air ventilation hole have been proposed (for example, refer to Patent Documents 1 and 2)
However, the venting apparatus or the like causes problems in their durability and in extremely deteriorated ventilation efficiency. This is because impurities generated during the tire vulcanization tend to remain between a tubular cylinder fitted into the vent hole, and a valve fitted into the ventilation hole of this cylinder when the remaining air and gas generated during the vulcanization are vented.    [Patent Document 1] Japanese patent application Kokai publication No. Hei 9-314567    [Patent Document 2] Japanese patent application Kokai publication No. 2001-232642